It often happens that there is a need to encipher information constituted by a finite sequence of symbols belonging to a finite alphabet, and that it is desired that the enciphered information itself be represented in the same alphabet and be of the same length as the initial sequence in the clear, i.e., the original information to be enciphered. This need is felt, for example, in respect of telephone numbers, virtual prepaid card numbers, bank card numbers, or indeed alphanumeric license plate numbers. In the first three cases mentioned above, the alphabet is constituted by the digits 0 to 9, whereas in the last case it is constituted by upper and lower case letters and digits.